A significant problem which arises is the very accurate positioning of the image sensor (an integrated circuit chip) with respect to the optics of the camera which focuses an image onto this sensor.
In an ideal situation, it is necessary for the plane of the integrated circuit chip to be perfectly perpendicular to the optical axis, and, assuming this condition is realized, it is necessary on the one hand for the active surface of the image sensor to be in the focal plane of the chip (Z-wise alignment), and on the other hand for the transverse and longitudinal axes of the chip (in the plane of the latter) to intersect on the axis of the optical system of the camera (X-wise and Y-wise alignment).
The present invention is concerned firstly with the X-wise and Y-wise alignment, but also with the Z-wise alignment. The positioning of the image sensor with respect to the camera is in principle done by mechanical means which take the package of the chip and place it as accurately as possible with respect to the camera, both X-wise and Y-wise (in a “horizontal” plane which is the plane of the surface of the chip) and Z-wise (in a “vertical” direction perpendicular to the surface of the chip). The error margin related to the means of mechanical positioning of the package is from one to two hundred micrometers. It depends mainly on the dimensional tolerances of the package. But this error margin assumes that the chip is perfectly positioned (in general centered) X-wise and Y-wise in its package, and at a height that is well known Z-wise with respect to the lower surface of the package.
To center the chip in the package, and at the same time to solder connection wires between the chip and the package, machines for encapsulation within packages generally use optical positioning by virtue of indexing marks formed on the package. The machine grasps the chip, detects the position of the marks, and places the chip at a location centered with respect to the marks. This optical positioning is necessary in particular so as to contrive matters such that the connection wires between the chip and the package are correctly put in place and soldered to the respective metal connection pads of the chip and of the package. These indexing marks are used in particular on ceramic packages, made by superposition followed by firing of multiple layers of green ceramic coated with silk-screen-printed metallic depositions; indexing marks are used for all sorts of integrated circuits, that is to say not only on image sensors. So that the positioning of the chip with respect to the connection pads of the package is optimized, the indexing marks generally consist of metallized zones forming part of the same metallization layers as the connection pads of the package; the marks are therefore silk-screen-printed at the same time as the connection pads and other conductors, if any.
But though the position of the marks is fairly accurate with respect to the pads of the package since they are silk-screen-printed at the same time, with however a certain inaccuracy resulting from the mode of fabrication of the packages based on the firing of green ceramic, it is not very accurate with respect to the edges of the package since the silk-screen printing is not referenced with respect to the edges of the package given that it is carried out before cutting the ceramic layers into individual packages. This results from the processes for fabricating these packages; typically, a ceramic package is made by superposing sheets of green ceramic silk-screen-printed with conducting pastes; the superposed stack is thereafter cut up into individual packages which are fired at high temperature. The position of the cutting of the stack with respect to the indexing marks is not sufficiently accurate because of the firing operation which globally reduces the dimensions of the layers in a proportion of about 15%. The tolerance in relative position of the cutting patterns or of the silk-screen print patterns of the layers after firing is not better than 150 micrometers, or indeed even less for large packages, and this tolerance is added to the global error in positioning the chip when the final accuracy of positioning of the chip with respect to the camera optics is evaluated.
This insufficiently accurate positioning of the chip with respect to the camera optics demands an expensive manual adjustment, for example a final position adjustment by micrometric screws before definitive gluing of the package of the chip in the camera.